Marine Biology 107, 225-233 (1990) Marine :==:Biology © Springer-Verfag 1990 Peribuccal organs of Placopecten magellanicus and varia (: ): structure, ultrastructure and implications for feeding II. The lips *

P.G. Beninger 1, 2, M. Le Pennec 1 and M. Auffret 1 1 Laboratorie de Biologic Marine, Facult6 des Sciences, Universit~ de Bretagne Occidentale, F-29287 Brest c6dex, France 2 D~partement de Biotogie et Centre de Recherches et d'Etudes sur l'Environnement, Facult6 des Sciences et de G~nie, Universit~ de Moncton, Moncton, New Brunswick E1A 3E9, Canada

Date of final manuscript acceptance: June 1, 1990. Communicated by R. O'Dor, Halifax

Abstract. In order to gain a better understanding of the bivalve feeding. Although such studies reveal the charac- roles of the peribuccal organs of in feeding, the teristics of feeding phenomena, they do not elucidate the structure and ultrastructure of the arborescent lips of mechanisms and effectors of such phenomena. In addi- Placopecten magellanicus and Chlamys varia were exam- tion, such studies may suffer from several sources of ex- ined using histological and electron-microscope tech- perimental bias (see Foster-Smith 1978). In order to niques. The anatomical and histological characteristics of achieve a more complete interpretation of bivalve feed- the lips suggest that they are closely related to the labial ing, structural and ultrastructural data are therefore nec- palps. The lips are formed of a densely-ciliated, ramified essary. oral epithelium and a sparsely-ciliated epithelium which Compared to the extensive literature concerning the links the ramified ciliated ridges and entirely constitutes structure and function of gills in bivalve nutrition (see the aboral surface. The differential distribution of the Beninger et al. 1988 and Le Pennec et al. 1988 for refer- three epithelial cell types (ciliated, non-ciliated, and mu- ences), relatively few studies have been performed on the cocytes), as well as the ultrastructural characteristics of peribuccal organs, i.e., the labial palps and lips. Details of the ciliated and non-ciliated cells and the secretions of the the structure and ultrastructure of the labial palps of the mucocytes suggest that the arborescent lips play at least scallops Placopecten magellanicus and Chlamys varia four different roles in feeding: (1) as a mechanical shield (family Pectinidae) and an interpretation of their func- to prevent the loss of mucus-bound food material in the tions are presented in the companion study immediately buccal region due to muscular-driven cleansing and preceding this contribution (Beninger et al. 1990). De- swimming currents; (2) as a trap for food material which spite their strategic location and their visibly complex is lifted out of the oral groove due to especially strong anatomy, the lips of pectinids have been the object of muscular-driven currents; (3) the consolidation of mucus scant and sporadic study, and most of this has been of a cords from the strands arriving from the palps and of functional nature, giving rise to conflicting interpreta- those which have been disturbed by the cleansing cur- tions (Yonge 1967, Bernard 1972, Gilmour 1964, 1974, rents; (4) the absorption of any dissolved and colloidal B. Morton 1979). Moreover, an accurate evaluation of matter which may dissociate from the mucus-particle the roles of the lips in feeding has been hampered by the masses in the buccal region. No anatomical detectors or lack of detailed information concerning their anatomy. effectors were observed which would suggest a role in Although some histological information exists (Gilmour particle selection. 1964), this is far from complete, and to the best of the authors' knowledge, no ultrastructural studies have been performed to date. Recent research on the structure and ultrastructure of the labial palps of P. rnagellanicus and Introduction C. varia has revealed potential accessory roles in feeding and possibly secretion (Beninger et al. 1990); it is there- In recent years, through indirect techniques such as stud- fore of interest to perform such studies on the lips of these ies of particle clearance, stomach-content analysis and two species. pseudofeces-feces analysis (Hughes 1975, Kiorboe and A number of monomyarian bivalves, including the Mohlenberg 1981, Shumway et al. 1985), important con- Pectinidae, present arborescent lip structures, while other tributions have been made to the understanding of mono- and dimyarians display non-arborescent lips which are developed to various degrees and arranged in * Please address all correspondence to Dr. Beninger at his Cana- configurations of varying complexity. Functional inter- dian address pretations of these structures have sometimes failed to 226 P.G. Beninger et al. : Lips of Placopecten and Chlamys

Fig. 1. Placopecten magellanicus. Gross morphology and anatomical relationships of gill (g), labial palps (lp), upper lips (ul), lower lips (11), mouth (m), and oral groove (og); lips and labial palps have been pulled apart to reveal oral region, t: lip trunks distinguish between complex configurations of otherwise General topography and anatomical relationships hypertrophied lips and true arborescent lips (Gilmour 1974). The lips of pectinids will herein be referred to as The lips arise as ramifications of the anterior margins of arborescent lips of the chlamid type (Bernard 1972). the labial palps, and also as independent, branched lobes The present study investigates the detailed structure of the upper and lower peribuccal surfaces (Fig. 1). The and ultrastructure of the arborescent lips of Placopecten upper and lower distal lip extremities often interdigitate magellanicus and Chlamys varia, and examines the impli- in the relaxed state, forming a frilled hood over the cations of such anatomical data for feeding in these two mouth. species. The lips are composed of two different types of ep- ithelia, which are strikingly similar to those of the labial palps (Fig. 2A, D). In those parts which arise as ramifica- Materials and methods tions of the labial palps, the ridged and smooth palp surfaces are continuous with the oral and aboral lip sur- The normally-expanded lips of Placopecten magellanicus and Chlamys varia contract rapidly in response to adverse tactile or faces, respectively. In those parts of the lips which arise as chemical stimulation (e.g. dissection and fixation). The following independent protuberances around the mouth, the spa- standard techniques of narcotization were attempted on several tial organization of the two epithelia is somewhat more occasions without success: progressive lowering of water tempera- complex. At the base of the lip trunks, the oral surface is ture to freezing; progressive addition of MgC12, MgSO 4, ethanol, entirely ciliated, while the aboral surface is sparsely ciliat- propylene, phenoxetol, methanol and xylocaine. Further trials us- ed from the base to the distal extremities. As the protu- ing chemical narcotizers were abandoned, since there is a real pos- sibility of inducing structural artefacts, such as those produced with berances ramify distally, the sparsely-ciliated epithelium tricaine methanesulfonate (MS-222 Sandoz) (Speare and Ferguson joins the ciliated ramifications of the oral surface, creat- 1989). Although it is more difficult to deduce the spatial organiza- ing a complex webbed structure. Hence, at the distal lip tion of the lips in the contracted state, this problem is not insur- mountable. Sampling, maintenance, and preparation of specimens for his- tology, scanning and transmission electron microscopy is described Fig. 2. PIacopectenmagellanicus. (A) Light microscopy of histolog- in the preceding paper (Beninger et al. 1990). ical section of distal extremities of lips; irregularly-folded ciliated ridges (cr) are joined by sparsely-ciliated oral (soe) and aboral (sac) epithelial tissue flaps; modified Masson trichrome (scale bar= 40/~m). (B) SEM of lips, showing transition between aboral sparse- Results ly-ciliated epithelium (sac) and ciliated ridges (cr) of oral lip surface (scale bar = 1 #m). (C) SEM of distal extremity of lips, showing The structural and ultrastructural characteristics of the extensive and dense eiliation of ciliated ridges (scale bar = 25/~m). lips of both species were found to be nearly identical, (D) Semithin section of lips, showing pigmented, sparsely-ciliated hence the following detailed description for Placopecten aboral epithelium (sac), non-pigmented sparsely-ciliated oral ep- magellanicus applies equally to Chlamys varia; any excep- ithelium (soe), ciliated ridges (cr) and cilia (c); toluidine blue (scale bar = 20 #m). (E) TEM basal region of ciliated oral epithelium; note tions are explicitly noted. The anatomical terminology multi-layered basal lamella (bl) and indentations of cell membrane used in the present work derives from previous studies of (arrowed); in sub-epithelial region, smooth-muscle fibres (mr) in- the lips and labial palps, notably by Gilmour (1964, serting into basal tamella, and haemoeytes (h) are visible (scale 1974), Bernard (1972) and Beninger et al. (1990). bar = 2 #m) P.G. Beninger et al. : Lips of Placopecten and Chlamys 227 228 P.G. Beninger et al. : Lips of Plaeopeeten and Chlamys

Fig. 3. Placopecten magellanicus. (A) Histochemical localization of density of cilia (c), with deep ciliary roots (cr) and associated mito- mucocytes (arrowed) in distal region of lips; Alcian blue and trioxy- chondria (m) at apical pole; lysosomes (ly) and phagosomes (p) of hematein (scale bar = 20/~m). (B) SEM showing intense mucus se- various types are scattered throughout supranuclear region; n: nu- cretion at ciliated base of oral lip epithelium; incomplete rinsing cleus; uc: undifferentiated cell (scale bar = 1/~m). (E) Semithin sec- prior to critical-point drying allows retention of some mucus balls tion of sparsely-ciliated epithelial tissue flaps joining adjacent ciliat- on epithelial surface (scale bar = 5 #m). (C) Detail of histological ed ridges. Three epithelial cell types are visible: simple (non-ciliated) section of ciliated oral epithelium of lip bases, showing numerous epithelial cells (sc), mucocytes (mc), and infrequent ciliated cells mucocytes (arrowed) and their secretions; Alcian blue and trioxy- (cc); note pigment granules in non-ciliated cells of aboral epithelial haematein (scale bar = 4 #m). (D) TEM of apical region of ciliated surface (sae); toluidine blue (scale bar = 15/~m) oral epithelium; note numerous apical microvilli (my) and high P.G. Beninger et al. : Lips of Placopecten and Chlamys 229 extremities, histological sections show oral ciliated ridges (Fig. 3 E), whose secretions are comprised entirely of neu- interrupted by sparsely-ciliated lamellae (Figs. 2A, D; tral mucopolysaccharides. Electron-clear ciliated cells are 3 E). However, only the aboral surfaces of the sparsely- sparsely distributed among the mucocytes and non-ciliat- ciliated epithelium are variably pigmented in some Pla- ed epithelial cells (Figs. 3 E; 4 B). The apical pole of the copecten magellanicus, while no pigment was present in ciliated and non-ciliated cells is characterized by exten- any of the Chlamys varia examined. sive microvilli forming a brush border with a glycocalyx, At the distal extremities of the branched lobes, the while the basal pole presents a thin, highly-convoluted heavily-ciliated epithelium is folded to such a great extent basal lamella (Fig. 4A). Lysosomes and phagosomes are compared to the sparsely-ciliated epithelium (its corre- very abundant at the apical pole of the sparsely-ciliated sponding "smooth" surface, as in the labial palps), that it epithelium of the oral surface (Fig. 4A), but not on the may be the only surface visible from the exterior; it thus aboral surface (Fig. 4B). Membrane-bound dark- constitutes both the oral and outer surfaces in these re- pigment granules resembling melanosomes are visible in gions (Fig. 2 C). The designation of the lip surfaces as the non-ciliated cells of the sparsely-ciliated epithelium of "inner" and "outer" (e.g. ciliated inner surface, smooth the aboral surface of the lips of Placopecten magelanicus outer surface) is therefore misleading, and it is preferable (Figs. 3 E; 4B); these are similar to those found in the to designate epithelial types and their orientation with smooth epithelium of the labial palps, and are thus re- respect to their origins in the peribuccal region (i.e., oral sponsible for the variably dark coloration seen in the lips and aboral). of this species. No such pigment was observed in the corresponding cells of the Chlamys varia specimens ex- amined. Microanatomy and ultrastructure of the epithelia No sensory cells were observed in the oral epithelium of any of the 37 sections examined using the Mann- Histological sections reveal that the complex ciliated Dominici staining technique (Gabe 1968: p. 766-767; ridges of the lips may be viewed as having the same gen- p. 960-961), nor were any tufts of long sensory cilia evi- eral structure as a palp ridge which has been thrown into dent under the scanning electron microscope. None of the several irregular folds (Figs. 2A, D; 3 A). The similarity cells examined using transmission electron microscopy between the labial palp ridges and the lip ridges extends displayed ultrastructural characteristics of sensory cells to the tissue organization and cell types present. The found elsewhere (e.g. tentacles or abdominal sense organ, pseudo-stratified, columnar epithelium of this surface Moir 1977a, b; gill axis, Beninger et al. in preparation). rests upon a multi-layered basal membrane, and is com- posed of two cell types: ciliated cells and mucocytes. Oc- casional undifferentiated cells can also be observed Internal anatomy (Fig. 3 D). The ciliated cells are tall and slender, and are charac- The internal anatomy of the lips is similar to that of the terized by numerous cilia whose roots extend to the labial palps, with the obvious modifications of extensive supranuclear region of the cell (Fig. 3 D). A conspicuous ramification and folding. Beneath the epithelial cells is a layer of mitochondria is distributed among these roots at diffuse layer of smooth muscle (Fig. 4 D). The remainder the apical pole. The apical membrane forms a dense cov- of the internal structure is composed of lacunar vesicular- ering of microvilli in a brush-border arrangement. Sever- connective tissue; haemocyctes are often visible in the al lysosomes and phagosomes can be observed through- lacunae, indicating that the expansion of the lips is ac- out the supranuclear region of the cells (Fig. 3 D). The complished via positive pressure within the haemocoel. cell membrane at the basal pole presents tall indentations Contraction of the lips is probably effected by the which extend well into the cytoplasm (Fig. 2 E). smooth-muscle fibres (Fig. 4 C, D). It may be assumed The tall, slender mucocytes of the heavily-ciliated ep- that, in the relaxed state, the lips and their branched ithelium are irregularly distributed over the oral surface, extremities are much less folded than they appear after being most numerous at the trunk bases in proximity to dissection and fixation. the mouth and relatively rare at the distal extremities (Fig. 3 A, B, C). The mucocytes at the trunk bases are very active (Fig. 3 B, C), whereas those at the distal ex- Discussion tremities appear to be relatively inactive (Fig. 3 A; weak Alcian blue colorations). The secretions of the ciliated Previous authors have suggested that in species possess- ridges of the oral epithelium consist of either acid muco- ing arborescent lips, these are entities distinct from the polysaccharides, neutral mucopolysaccharides, or of labial palps, and that they do not share a common embry- both together (Alcian blue - periodic acid-Schiff reac- ological origin (see Bernard 1972). However, the great tion). degree of structural and histological similarity of these The sparsely-ciliated epithelium is cuboidal, and two organs, as well as the fact that part of the lips derives strongly resembles the smooth surface of the labial palps, directly from the anterior palp margin, with a continua- although the pronounced retraction of the lips upon dis- tion of the respective ciliated and smooth epithelia, tend section and fixing forces the cells to assume a pedunculat- to argue for a very close anatomical relationship. ed appearance (Fig. 3 A, E). On the aboral surface, the The observations of the present study on Placopecten abundant mucocytes consist of characteristic goblet cells magellanicus and Chlamys varia confirm the macroscopic 230 P.G. Beninger et al.: Lips of Placopecten and Chlamys P.G. Beninger et al. : Lips of Placopecten and Chlamys 231 organization of the lips as presented by previous Although this mode of particle ingestion is supported authors (Yonge 1967, Bernard 1972), and are at variance by direct observation of material prior to ingestion (see with the representation of Gilmour (1964), which showed Kellogg 1915, Bernard 1972, 1974, Gilmour 1974, Foster- the upper and lower lips forming a tube over the mouth. Smith 1975, 1978, Beninger et al. 1988), as well as by the Not only are the lips free, but their natural appearance anatomical data of the present study, it is at variance with resembles more a cauliflower than a tube. Kiorboe and Mohlenberg's (1981) observation that parti- The mucocytes of the ciliated ridges and their two cles in the oesophagus of Mytilus edulis were in free sus- different secretions are histologically and histochemically pension, whereas particles in pseudofeces were bound in identical with those of the labial palp ciliated-ridges, mucus. Moreover, as pointed out by Newell and Jordan whereas the goblet cells and their homogeneous secre- (1983), in their study on Crassostrea virginica, it is diffi- tions of the sparsely-ciliated epithelia are identical to cult to see how suspended particles could be ingested, those of the smooth palp surface (Beninger et al. 1990), given the capstan mechanism of the crystalline style (Pur- suggesting a similarity of mucus function for the corre- chon 1977, J. E. Morton 1979). Similarly, histological sponding surfaces of these two peribuccal organs. observations of the oesophageal epithelium of Pla- The differential distribution of mucocytes on the oral copecten magellanicus and Pecten maximus have shown it surface suggests a corresponding dichotomy of function to contain a very high density of active mucocytes (A. between the proximal and distal regions. The first of these Donval and P. Beninger, Laboratoire de Biologie Ma- would appear to be related to the preparation of material rine, Universit6 de Bretagne Occidentale, Brest, France, for ingestion. Particles arrive at the labial palps of unpublished observations). Bernard's (1974) report of bivalves bound in mucus cords (Kellogg 1915, Bernard particles bound in mucus in the oesophagus of Cras- 1972, 1974, Gilmour 1974, Foster-Smith 1975, 1978, sostrea gigas also conflicts with the observations of Beninger et al. 1988). As particle concentration in the Kiorboe and Mohlenberg. Preliminary results of histo- external medium increases, the cords are subjected to logical examination of the oesophagus of wild M. edulis mechanical handling by the ridged palp surface, during have confirmed the presence of abundant mucocytes, as which they may break into strands or agglomerates of well as mucus-bound particle masses (Beninger et al. in various lengths (Foster-Smith 1975, 1978). Although it preparation). It would thus appear that food material has not yet been demonstrated unequivocally that the ingested by bivalves is bound in mucus, rather than in labial palps are capable of extracting and selecting indi- free suspension; the lip bases add mucus to the arriving vidual particles from the mucus strands (Beninger et al. food material, thus consolidating the mucus strands, 1990), accepted material from the ridged palp surface will whose entry into the stomach is assisted by the rotating nonetheless arrive at the oral groove proximal to the crystalline style. mouth in the form of independent masses. The intense Various interpretations have been advanced concern- mucosecretory activity of the ciliated oral-lip surfaces at ing the role of the complex lip topography in feeding. the trunk bases may thus serve to consolidate this materi- Bernard (1972) stated that mucus-bound food masses al into a single mucus strand prior to entry into the from the labial palps travelled over the aboral lip surface, mouth. This would facilitate the passage of particles to where they were subjected to screening by the interdigita- the stomach, where the mucus strand would be drawn in tions of the upper and lower lips; small particles penetrat- and wound around the rotating crystalline style prior to ed through to the oral surface, while large particles and digestion (Purchon 1977: p. 225-228; J. E. Morton 1979: mucus masses were rejected as pseudofeces. Such obser- p. 114-117). The proposed role of the lip bases in consol- vations cannot be reconciled with the anatomical data of idating mucus strings does not account for their arbores- the present study. As previously shown, the ciliated sur- cent structure, and it is probable that even those bivalves face of the lips is a continuation of the ridged palp sur- possessing simple lips present such an adaptation of the face; for the mucus masses to be transferred directly to lip bases and peribuccal epithelium. Further study on the aboral lip surface they would have to change surfaces, other species is needed to test this hypothesis. and no possible anatomical effectors for this have been observed. In addition, the aboral lip surface is only sparsely ciliated, and is certainly not an epithelium spe- Fig. 4. Placopecten magellanicus. (A) (B) TEMs of sparsely-ciliated cialized for the transport of material. This proposed epithelium of lips. (A) General view of non-ciliated cells of sparsely- screening function therefore seems improbable, as point- ciliated oral epithelium, showing dense microvilli (mv) forming a ed out by B. Morton (1979). brush border with a glycocalyx (arrowed) as well as numerous The most convincing interpretation of the func- lysosomes (ly) and phagosomes (p) with electron-dense contents; bl: tional significance of the lips of scallops was first put basal lamella; ct: vesicular-connective tissue; n: nucleus; uc: undif- ferentiated cell (scale bar = 1 #m). (B) TEM showing membrane- forward by Yonge (1967) and later supported by B. bound dark-pigment granules (pg) of various electron densities in Morton (1979). True complex lips of the chlamid non-ciliated aboral epithelial cells, c: cilia; cc: ciliated cell; mc: mu- and limid type are only present in those monomyari-" cocyte; mv: apical microvilli (scale bar= 1 #m). (C) TEM from ans which have evolved a mechanism of expelling internal region of lips ; connective cells (coc) and muscle fibres (mf) pseudofeces from the mantle cavity via periodic move- are scattered among loose array of collagenous fibres (cO (scale ments of the valves, effected by the adductor muscle bar= 1 #m). (D) Histological section of part of ciliated ridge, showing extensive layer of smooth-muscle fibres (ml) beneath (i.e., the Pectinacea and Limacea; all other monomyari- basal lamella (bl); modified Masson trichrome technique (scale ans and dimyarians expel pseudofeces via ciliary tracts bar = 20 #m) and cilia-driven currents). Muscle-driven currents exit at 232 P.G. Beninger et al. : Lips of Placopecten and ChIamys the antero-dorsal and postero-dorsal mantle margins; the The ultrastructural data for the ciliated and non- anterior current would thus tend to expel the food-laden ciliated epithelial cells of both the oral and aboral surfaces mucus strings in the buccal region prior to ingestion. The is highly suggestive of a role in the absorption of dis- presence of a protective hood such as that afforded by the solved matter; furthermore, both the densely-ciliated and interdigitating arborescent lips of the Pectinacea or the the sparsely-ciliated epithelium of the oral surface appear fused, spherical lips of the Limacea (B. Morton 1979) to be capable of absorbing colloidal matter, as has previ- would thus reduce the loss of food strings during clear- ously been reported for the ridged surface of the labial ance of pseudofeces. As has been noted for the smooth palps (Beninger et al. 1990). Such an alternative trophic surface of the labial palps, (see companion study immedi- function would be amplified by the extensive ramification ately preceding this contribution), the particularlY abun- of the lips. It is interesting to note that the aboral epithe- dant mucocytes of the aboral lip epithelium may serve to lium appears to have a lesser absorptive role than the oral reduce the frictional resistance at the epithelium-pallial epithelium, as was also the case for the labial palps fluid interface. This would consequently attenuate the (Beninger et al. 1990). The periodic increase in water flow adverse effect of the anterior cleansing current on the lips. in the buccal region brought about by the cleansing cur- It is interesting to note that the successful evolution of rents would tend to dissolve some of the mucus and disso- the muscle-driven cleansing current, conditioned in part ciate some of the smallest particles prior to ingestion, by the simultaneous evolution of the complex lips, was an whereas the ramified oral epithelium would recover most important factor in the evolution of swimming behavior of the material that might otherwise escape ingestion. in those Pectinacea and Limacea which are capable of This constitutes a fourth possible role for the lips, sup- such behavior (Yonge 1967). Indeed, when the violent ported by ultrastructural data. valve-snap response occurs, the lips are contracted The arborescent lips of scallops thus appear to be around the mouth region, offering an even greater degree structures which fulfill several key functions in the inges- of protection from the currents so generated. tion of food material, as well as possessing a possible The above interpretation of the role of the arbores- alternate trophic role which is amplified by their highly cent lips does not account for the extensive ciliation of the branched topography. Further information is needed ridges of the oral surface. The ciliation of the ridged oral concerning the histology and ultrastructure of the lips of surfaces, from the bases to the distal extremities, is as non-pectinaceans, in order to achieve a more comprehen- dense as that observed for the ridged surface of the labial sive understanding of lip function in the Bivalvia. palps of Placopecten magellanicus and Chlamys varia in The detailed structural and ultrastructural observa- the present study. This extensive ciliature could accom- tions of the lips reported here, combined with similar plish either or both of two functions: generation of water studies of the gills (Beninger et al. 1988, Le Pennec et al. currents, and transport of solid material. It is unlikely 1988) and labial palps (Beninger et al. 1990) clarify the that these cilia function to create water currents, as these problems and provide baseline data necessary for the would have a negative effect on the stability of the mucus elucidation of the mechanisms of scallop feeding phe- cords being ingested. A much more plausible function for nomena such as particle capture and selection. Although these cilia would be to trap any food material that, in complete resolution of these fundamental questions has spite of the protection afforded by the lips, is lifted out of not yet been achieved, recent and current progress in this the oral groove in the buccal region during muscular field is encouraging (see Beninger 1990 for review). cleansing movements. Support for this interpretation comes from the histological similarity of the mucocytes Acknowledgements. The authors wish to thank Dr. G. Sinquin for and their secretions with those of the ciliated ridges of the assistance with electron microscopy, Dr. K. Benhalima for help labial palps, which are specialized in particle-handling. with histology, and MM. A. Le Mercier and L. Blanchard for their excellent photographic work. Fig. I was expertly drawn by Ms. L. The lips of scallops would therefore serve at least three Colpitts. Earlier versions of the manuscript were strengthened after functions with respect to the ingestion of food material: being commented upon by Drs. R. L. Foster-Smith, Y. Poussart, S. (1) as a mechanical shield to prevent the loss of mucus E. Shumway, and Ms. R. Friedrich. The skillfulword-processing by cords in the buccal region, due to the anterior cleansing Mine J. Corvest and L. Briard is gratefully acknowledged. Speci- current; (2) as a trap for food material which is lifted out mens of Placopecten magellanieus were collected using the facilities of the Huntsman Marine Science Centre, St. Andrews, New of the oral groove due to especially strong muscular- Brunswick, Canada. This collaborative study was funded by the driven currents (the extreme being swimming behavior); CNRS/CNRC scientific exchange programme, as well as by Natu- (3) the consolidation of mucus strands from the cords ral Sciences and Engineering Research Council Grant No. 0003658, arriving from the labial palps and of those which were Facult6 des l~tudes Sup~rieures et de la Recherche de l'Universit~ de disturbed by cleansing currents, trapped by the lip oral Moncton Grant No. 20-53202-54-111, and Department of Employ- cilia, and transported back to the oral grooves. ment and Immigration Challenge 1988 and Challenge 1989 pro- The apparent absence of sensory cells on the oral grammes. epithelium of the lips mirrors the results obtained for the labial palps (Beninger et al. 1990). More detailed research Literature cited will be required to determine whether specialized or "cryptic" sensory cells are present on the lips; however, Beninger, P. G. (1990). Structures and mechanisms of feeding in these preliminary results suggest that the lips have no scallops: paradigms and paradoxes. In: Shumway, S. E, (ed.) major sensory role and thus are not involved in particle An international compendium of scallop biology and culture: a selection. tribute to James Mason. J. Wld Aquacult. Soc. (in press) P.G. Beninger et al. : Lips of Placopecten and Chlamys 233

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